Field of the Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic and other apparatus, a number of movable structures or components are driven by electromagnetic actuators for obtaining a fast and accurate position, speed, acceleration, jerk, snap, etc. of each moving component. An example of such a moving component in a lithographic apparatus is a reticle mask, a reticle stage for supporting a reticle, or a wafer stage for supporting a substrate.
Linear electromagnetic actuators, i.e., electromagnetic actuators performing a linear movement, sometimes utilize an iron core concept having a stationary part manufactured at least partly from a magnetized or magnetizable material interacting with at least one movable electromagnetic coil being connected to a component or structure to be moved. The coil may be wound around a core of a magnetized or magnetizable material. By energizing the coil or coils in an appropriate way with an electric current, the coil or coils move in a predetermined direction relative to the stationary part. A position, speed, acceleration, jerk, snap, etc. of the moving coil or coils and its associated component may be generated by selecting and shaping the current within the design boundaries. A one-phase or three-phase or multiple-phase current and coil system may be employed.
When the coils are moved, power cables for supplying current, sensor lines for providing sensing signals, air supply ducts for air bearings and/or cooling water ducts must be movable also, since they must be connected to the moving coil(s) and/or component or structure. Such appending cables, lines and ducts add to the moving mass, are generally difficult and expensive to manufacture, to install and to maintain, may cause life-cycle problems and to a certain extent may cause disturbance forces acting on the movable component or structure.
In new generations of lithographic and other apparatus, dimensions of components increase, and thus masses and volumes of components increase. Also, it is desirable to increase acceleration and deceleration of components in order to raise the productivity of the apparatus. For higher masses combined with higher acceleration/deceleration, higher coil currents are needed which generally require a heavier cabling and increased sizes of air supply and/or cooling water ducts as a result of higher dimensions, masses and dissipation. Bearing structures, such as air bearing pads and magnetic pretension plates, may become more extensive and complex to be able to cope with larger forces experienced during movement.
Iron core motors suffer from torque ripples, cogging (preferred positions in zero-current circumstances), K-factor (generated force per ampère) ripples, normal force ripples and other reluctance forces. One or more of these factors may be critical for machine performance. Since the torque ripple, the K-factor ripple and the normal force ripple each are dependent from the current amplitude, an increased current may lead to an increased torque ripple, K-factor ripple and normal force ripple.